Add shop, POS sync, event ticketing, and shipping catalog apps (#6)

## Summary
- Closes #5
- Optional `shop`, `pos_sync`, `events`, and `shipping` apps gated by `FEATURE_*` flags
- Deps match the catalog: shop/events need email + Stripe; POS/shipping need shop
- Portal inventory, POS webhooks, capacity tickets, EasyPost/Pirate Ship shipping

## Test plan
- [ ] `manage.py test` (152 passed locally)
- [ ] Shop cart + paid order decrements stocked inventory
- [ ] POS inbound webhook decrements SKU; paid order queues outbound reserve
- [ ] Event capacity blocks overbook; paid order emails ticket codes
- [ ] Shipping stub label + Pirate Ship CSV of unshipped paid orders
- [ ] `validate-env.sh` rejects shop without email/payments, POS/shipping without shop

Reviewed-on: #6

scripts/validate-env.sh

@@ -82,6 +82,41 @@ if _true "${FEATURE_SOCIAL_AI:-}"; then
     exit 1
   fi
 fi
+if _true "${FEATURE_SHOP:-}"; then
+  if ! _true "${FEATURE_EMAIL_SMS:-}"; then
+    echo "FEATURE_SHOP requires FEATURE_EMAIL_SMS" >&2
+    exit 1
+  fi
+  if ! _true "${FEATURE_PAYMENTS:-}"; then
+    echo "FEATURE_SHOP requires FEATURE_PAYMENTS" >&2
+    exit 1
+  fi
+fi
+if _true "${FEATURE_POS_SYNC:-}"; then
+  if ! _true "${FEATURE_SHOP:-}"; then
+    echo "FEATURE_POS_SYNC requires FEATURE_SHOP" >&2
+    exit 1
+  fi
+  if [[ "$DJANGO_ENV" == "prod" || "$DJANGO_ENV" == "beta" ]]; then
+    required_vars+=(POS_WEBHOOK_SECRET)
+  fi
+fi
+if _true "${FEATURE_EVENTS:-}"; then
+  if ! _true "${FEATURE_EMAIL_SMS:-}"; then
+    echo "FEATURE_EVENTS requires FEATURE_EMAIL_SMS" >&2
+    exit 1
+  fi
+  if ! _true "${FEATURE_PAYMENTS:-}"; then
+    echo "FEATURE_EVENTS requires FEATURE_PAYMENTS" >&2
+    exit 1
+  fi
+fi
+if _true "${FEATURE_SHIPPING:-}"; then
+  if ! _true "${FEATURE_SHOP:-}"; then
+    echo "FEATURE_SHIPPING requires FEATURE_SHOP" >&2
+    exit 1
+  fi
+fi
 
 missing=()
 for var in "${required_vars[@]}"; do